The metal industry continues to require new materials for fabricating products that are improved in performance and are less costly to manufacture. Because of the vast differences in the characteristics of metals themselves, some materials are uniquely adapted for special uses. Steel, for example, has a high characteristic tensile strength and is easily formable in sheet form and thus is well adapted for stamping automobile body parts as well as a host of other commercial and consumer goods. However, steel has a high density and is not suitable for lightweight applications such as those in the aerospace industry. Aluminum, on the other hand, is light weight, but has a lower tensile strength, as compared to steel, and is not easily formable in sheet form, and is thus not well adapted for use in stamping automobile body parts. When stamping contoured parts, the sheet aluminum material becomes thinned and even breaks at the high stress locations, such as areas where sharp curves and corners are formed. Because of the requirements for higher strength and light weight materials in many modern applications, titanium has become a material of choice, especially in the aerospace industry, because of its high strength and light weight properties. The demand for higher strength and lower weight materials continues to grow and is becoming very important not only in aerospace industry but also in automotive industry. The use of high strength and low density materials in the automobile industry is becoming extremely important because of more stringent requirements to control environmental pollution and to conserve the fossil energy resources.
A relatively new process has been developed for increasing the tensile strength of aluminum, or other soft metals, in an attempt to fulfill the current and future demands for high strength and low density materials, while yet being easily formable in many metal-forming areas. The tensile strength of metals can be increased by many methods, one being a process by which the grain size of the metal is reduced and made very small. With a smaller grain size, the hardness and tensile strength of the metal is increased without compromising the ductility properties. The reduction in the grain size of a metal or alloy can be achieved by thermomechanical processing (TMP) where the material undergoes an extremely high degree of deformation. It is well known that when a metal undergoes severe thermomechanical deformation, the grain structure becomes smaller, and the material becomes correspondingly stronger at low temperatures. Many metal processing techniques are known which provide extremely large material deformations, including the well-known TMP techniques, the torsional/pressure technique, extrusion, and others. While yet in an experimental stage, softer metals can be hardened by undergoing a process called Equal Channel Angular Extrusion (ECAE), which is also known also Equal Channel Angle Pressing (ECAP). Because the processes are substantially identical, except for name, the process is referred to herein as the ECAE/P process. The ECAE/P process reduces the grain size of the metal by forcing the material through an angled die so that the metal undergoes a shear deformation without a corresponding change in the cross-sectional size thereof. A number of stages can be utilized so that the billet undergoes a shear deformation along different axes of the billet. This sequential shear deformation in the material can result in an ultrafine grain size, on the order of a few microns, or less. For a better understanding of the ECAE/P process, reference is made to the following U.S. patents: U.S. Pat. No. 5,620,537 by Bampton; U.S. Pat. No. 5,809,393 by Dunlop, et al; U.S. Pat. No. 5,826,456 by Kawazoe, et al; U.S. Pat. No. 5,904,062 by Semiatin, et al; and U.S. Pat. No. 6,197,129 by Zhu, et al. The ECAE/P process is well adapted for use with softer metals such as aluminum, copper, magnesium, nickel, titanium, and their corresponding alloys, and others. The shear strain to which these materials are subjected during the ECAE/P process increases the hardness thereof. These metals can thus be used in many other applications which heretofore rendered them unacceptable.
FIG. 1 illustrates, in a generalized manner, how billets are work hardened through the use of an ECAE/P die and a ram. The die 10 is constructed in a conventional manner with die steel or other suitable materials. Formed in the die 10 is an entry channel 12 and an exit channel 14. The ratio of the diameter or side of the channel cross sections to the respective length of the channels is typically in the range of 1:4 to 1:8. The entry channel and exit channel are not colinear, but rather are formed at an angle Φ with respect to each other. As the die angle Φ becomes smaller, more shear is imparted to the billet 16. In addition, the channels 12 and 14 are substantially identical in cross-sectional size and shape, and thus the billet 16 being processed does not change in cross-sectional shape as it is moved through the die 10. The principle of operation of the ECAE/P technique is that as the billet 16 is forced through the angled portion of the channel, where the entry channel 12 joins the exit channel 14, the billet undergoes a severe plastic deformation. Repeated deformation of the material through the die causes the grain structure to become smaller, thereby increasing the hardness of the billet 16.
In a conventional process, the billet 16 is pushed through the die 10 by a hydraulic ram 18. As can be appreciated, the length of the billet must be somewhat short so that the billet does not buckle at the entrance of the entry channel 12. Billet cross sections on the order of about 1 inch to 2 inches in diameter or side dimensions have been processed through ECAE/P dies in this manner. With a limitation of short billets, in connection with the diameter/length ratios noted above, there is inherently a substantial amount of waste associated with the process, it being realized that the frontal end and rear end parts of each billet may be unusable. The ECAE/P method of work hardening a metal is thus acceptable for short billets. Hence, where the fabrication of large metal work pieces is necessary, the use of ECAE/P processed metals is not presently economically feasible.
It can be seen from the foregoing that a need exists for a process that can produce long billets of metallic materials using ECAE/P methods. Another need exists for a metal processing system that can produce large quantities of ECAE/P-hardened metals, with substantially lower energy requirements for carrying out the process. Yet another need exists for a method of continuous processing of long metal billets through successive ECAE/P dies to thereby achieve large quantities of ultrafine grain, hardened materials adapted for new and existing uses. Another need exists for a process where ultrafine grain materials can be produced by severe plastic deformation techniques, with less waste.